Best squirrel rifle caliber

I'm planning to upgrade my state-land legal squirrel setup. Not going nuts, probably a $500 package. I'm thinking a Ruger American with a Nikon Prostaff on it. Perhaps not those exactly, but in that class level somewhere.

What's your preference for caliber, and why? I'm open to whatever rimfire. .17s up to a .22 mag.

Anything you choose will be fine as long as you're comfortable with it and it is sighted in properly. The rugers are great little rifles. I hunt squirrel with probably the cheapest .22 around. Its a Mossberg 802 with a cheap Nikon and it gets the job done just fine. I think with the scope and then extra mag was only maybe 250. So really it's up to you, they all work well when sighted in.

Honestly, I prefer a PCP Air rifle for squirrel hunting, just my .02. I have done quite well in MA with mine this year so far, next time I go out I will have to take some pics for the squirrel competition. Though mine cost quite a bit more then $500 there are some very good ones on the market that can be had for that price. My .25 Benjamin Marauder is more accurate than almost any .22 rimfire I've shot, out to 175yards (And I know I could push it further if I wanted to sacrifice shot count per fill) Not that I shoot at squirrels anywhere near that distance, but at 75 yards I can lay pellet on top of pellet all day long. Pellets are dirt cheap and easy to find, and the air rifle is backyard friendly so you can easily stay in practice with it year round. The other benefits I've found are they are very quiet so you may get a followup shot that you otherwise wouldn't with a rimfire. Also if you hunt other states like I do, most don't recognize them as a firearm and the 500ft rule does not apply, and people I've found are a lot more likely to let you hunt on their property with an air rifle then with a rimfire. Again, I know you asked about rimfires but maybe just something to think about.

I used a .22 for many, many years. 2 years back, I switched to a 20 gauge and my harvest numbers jumped considerably. Usually shooting canopy/tree tops and often they are moving, found that the .22 or even my .17 HMR just wasn't getting it done consistently.